It should not be assumed that any of the approaches described in this section qualify as prior art merely by virtue of their inclusion in this section.
Pulse oximetry is a method for estimating blood oxygen saturation by utilizing specialized light sources and optical sensors. Tuned light wavelengths are either transmitted through or reflected from a human tissue and are used to estimate a relative proportion of oxygenated blood. This estimated oxygen saturation, termed SpO2, strongly correlates to arterial blood oxygen saturation. One of the advantages of pulse oximetry over other methods of determining oxygen saturation, such as blood sampling, is that the pulse oximetry is non-invasive, minimally intrusive, generally not painful, portable if it needs to be, and provides for continuous readings.
For a healthy human at normal altitudes, SpO2 is typically 95% or above, with 90% or below indicating hypoxemia, and sustained periods of 80% or below possibly indicating serious medical complications. SpO2 can reflect statuses of individuals suffering from various clinical disorders such as Chronic Obstructive Pulmonary Disease (COPD) or asthma, whether in a stable chronic condition or during an acute phase. Pulse oximetry is also useful in neonatal monitoring, surgical monitoring, or status evaluation when the possibility of oxygen depletion must be considered (pilot monitoring, deep sea diving, and so forth).
Certain clinical conditions can interfere with either the accuracy of pulse oximetry or affect interpretation of results. Diseases which affect peripheral circulation can make the SpO2 an inaccurate estimate of arterial oxygenation. For example, anemia impedes utilization of blood oxygen regardless of the saturation level.
Human activity and behavior can also affect results of the pulse oximetry measurements. Movement of the sensor used in pulse oximetry can interfere with signal acquisition. Temperature changes can affect blood flow to the area being monitored with the sensor. Sweating can affect optical quality. Smoking can increase carbon monoxide which competes with oxygen to bind hemoglobin and can confuse most systems. Contrast dye injections can interfere with blood optical qualities.
Pulse oximetry depends on differences in light absorbance characteristics of oxygenated hemoglobin (oxyhemoglobin) and non-oxygenated hemoglobin (deoxyhemoglobin). The former absorbs light at about 660 nm (in the visible red range) and the latter absorbs light at about 940 nm (infrared). Both light signals, whether reflected or transmitted, fluctuate with the arterial pulse. The resulting signals, photoplethysmograms (PPGs), can indicate volume changes due to blood flow. Pulse oximetry utilizes the intensity change (light signal fluctuation at each heartbeat) for each wavelength to eliminate the confounding optical effects of other tissues (which remain constant). SpO2 can be estimated using the Beer-Lambert Law, which relates to light absorbance due to the concentration of a substance in media, and empirically-derived reference curves from blood samples of hypoxic volunteers, based on the ratio of these changes in each wavelength (delta 660 nm/delta 940 nm), although other complex factors are often included in the calculations.
Typically, the light sources are light-emitting diodes (LEDs) optimized for output at each of the target wavelengths. A single optical sensor (often a photodiode) may be used for both. Each LED can be activated separately, and accompanied by a “dark” period where neither is on (to obtain ambient light levels). The sensor records light transmitted or reflected for each LED. The obtained signals can be processed in real time or offline.
The sensors can be utilized in either a transmission or a reflectance mode. In the transmission mode, the sensor is typically attached or clipped to a translucent body part (finger, toe, earlobe, and so forth). The LED light sources can be positioned on one side of a body part and the sensor can be positioned on the directly opposite side. The light passes through the entirety of the body part, from one side to the other, and is thus modulated by the pulsating arterial blood flow. In the reflectance mode, the light source and the sensor are on the same side of the body part (e.g. a forehead, finger, and wrist), and the light is reflected from the skin and the underlying near-surface tissues back to the sensor.
Despite the conceptually different optical paths in the reflectance pulse oximetry and transmission pulse oximetry, conventional transmission type signal processing can be used for determining of oxygen saturation. However, the sensor part may need to be adapted to enhance the reflectance signal and the usage of a transmission model for reflectance analysis can result in unstable and erroneous SpO2 estimates.
Continued monitoring of chronic outpatients can be greatly enhanced by accurate SpO2 measurements. A reflectance device can be worn on body parts such as a wrist or an ankle and would impose minimal burden on normal activities. Thus, developing reliable reflectance oximetry devices based on a specific light reflectance model holds great promise for outpatients suffering from chronic diseases.